Pantographing, as it relates to bicycles at least, is the engraving of a brand name, logo or shape into a component using a pantographing rig, which looks something like this:
From Nick Sherman
One arm of the machine follows a master pattern, and the other arm holds a tool which engraves the artwork onto the component. The engraved areas are usually painted a contrasting color. The resulting parts look like this:
Photo: Flickr user Raytracing
Related aesthetic modifications include milling, which usually looks like stripes:
Photo: Flickr user Styleman27
and "drillium," the colloquial expression for drilling the crap out of your bike parts.
Photo: Flickr user smarty-1
Here's a single assembly which has had all three of these techniques applied to it.
Photo: Flickr user sibkissolutions
The crank spider has been milled, as have the black-painted grooves in the outer chainring, which were then drilled. The "dP" logos are probably pantographed, although unusually for the technique, the work went all the way through the piece.
Editor's note: since initial writing, Randy has suggested that the dP logos may not be accurate to the strictest definition of pantographing, as they go all the way through.
Drillium was more about customizing than it was about reducing weight, and was usually home-made. Pantographing and milling require more elaborate tools and were usually done by machine shops, but most often, not the manufacturer of the parts themselves. There are exceptions of course; companies like Huret and Stronglight released factory-drilled versions of their derailleurs and chainrings to cash in on the trend. The drillium craze only lasted a few years starting in the early 1970s, and all of these folk-art techniques fell out of fashion in the early 1980s with the ascension of Japan as the leader of the industry. With renewed interest in bikes of this period, collectors and enthusiasts are again making these modifications to vintage parts, often far more elaborately than what was done in period.
Tip: Not all bicycle engraving is pantographing. Most details on seatstay caps and fork crowns are cast, not pantographed. Also, only metal is panto'd. I've seen numerous misuses of the word applied to plastic pumps and water bottles, all of which are stamped in some way. So, you might have a pantographed stem, for instance, but you probably don't have a panto frame, despite what that guy on eBay told you.
Editor's note: great thanks to Randy for writing this up. Also, gratitude to those Flickr users that so kindly agreed to be included in this article.